1. Field of the Invention
The present invention shows a plurality of different-sized metal laminates stacked for a built-in capacitor in the manufacturing process of a printed circuit board (PCB), which is used to reduce the error in process and raise the capability of noise-immunity in a PCB.
2. Description of Related Art
In the age of high frequency and high-speed electrical systems, the need for accuracy of electrical devices is increasingly acute. The manufacturing process is the focus of efforts to raise the precision of active and passive elements. More particularly, it is a severe challenge to the manufacturing process of capacitor in a PCB, which will be operated at high frequency and is small in size.
In the prior art, there are two main capacitor techniques; one is a discrete component capacitor using surface mount technology (SMT) as an example, the other is a newly developed capacitive substrate. The discrete component capacitor has at least three problems, including: (1) the range of working bandwidth is insufficient; (2) the alternating impedance is not low enough; and (3) the capacitor cannot be integrated into the system in package. Current capacitive substrate elements also have three main drawbacks, including: (1) the via parasite effect is excessive; (2) the electric design thereof is only two-dimensional and the area thereof is too large; and (3) the dielectric material has considerable electric loss. But the PCB with built-in capacitor in the invention can reduce the usage of passive elements for reducing costs, volume, and number of welding points, while raising the reliability and the electric characteristics thereof as well.
The printed circuit board in prior art is usually composed of flat substrates with the same dielectric coefficient; for instance, the Fiberglass Fabric (FR4) is used. However, the noise-immunity of this PCB at high frequencies is bad, and the low passive element integration is a demerit thereof. With further improvement as provided by U.S. Pat. No. 5,079,069 shown in FIG. 1, which is a capacitive laminate for use in a capacitive printed circuit board and method of manufacture, there is a plurality of electronic elements 12 set on a PCB 10. The active elements like ICs, transistors, and the passive elements like capacitors and resistances all can be installed on both sides of PCB 10, which generally is one or a plurality of laminates composed of power, ground or other conductors connected with each other.
In the prior art, the capacitor components are replaced by a built-in capacitive substrate as described in the following.
The role of current PCB is not only to be in charge of the function of, signal transferring as before, but also to integrate many passive or even active elements therein. Reference is made to FIG. 2, which is the cross-sectional view of a PCB fabricated by the built-in method. The surface-mounted device 20 is set on the surface of PCB 10. Power wiring 34 and ground wiring 36 are vertically connected to first plate 28 and second plate 30 of built-in capacitor 26 via conducting openings, respectively, which become the power and ground ends of the capacitor shown as the black spots in the drawing. Accordingly, the built-in capacitor 26 in PCB 10 has a distance between power and ground ends formed by first plate 28 and second plate 30, respectively, and a dielectric material placed between the plates is coupled. A signal wiring 38 passes through the PCB 10 and functions as the wiring of connection or signal transmission among other elements.
FIG. 3 shows a cross-sectional view of a multiple-layer built-in capacitor, which has upper and under side coupled with a built-in capacitor and provides double capacitance in this prior art embodiment.
The advantages of the built-in capacitor in the prior art are described as follows: (1) the interference of high-frequency noise is suppressed; (2) the number of connection layers required in PCB is reduced; (3) the density integrated with the whole system is increased and the area of the PCB is reduced. For raising the capacitive characteristics and restraining noise in the PCB, different dielectric materials are added in the inner layer of substrate in prior art. The main purpose of forming a built-in capacitor using a high dielectric coefficient substrate is to reduce the area of the PCB. Nevertheless, once the area is reduced, inaccuracy increases due to the alignment error in the manufacturing process of the PCB. If the precision of the manufacturing process is insufficient, inaccuracy is easily produced by compression alignment errors, and a difference between the two plates in the manufacturing process will not provide high-precision discrete capacitor.
In the top view of built-in capacitor shown in FIG. 4A, there are two parallel plates 28′, 30′ of a capacitor, which are like first plate 28 and second plate 30 shown in FIG. 2. The precision of manufacturing process is not high enough and results in an opposite slip between the two plates shown in drawing. Thus, only the shaded part area contributes to the capacitor, which is not the intended result. FIG. 4B is cross-sectional view of the two plates, which also shows the opposite slip therebetween.
The present invention thus employs different sized plates formed with non-symmetrical electrodes as the capacitive apparatus of built-in capacitor, where the bigger one will cover the smaller one to reduce the error compression alignment and improve the noise-immunity and efficiency of a capacitive substrate with a high precision capacitor.